Plasma etching techniques have been widely used for etching products such as semiconductor wafers. Basically such a technique comprises the exposing of wafers to a plasma to remove surface materials, such as silicon oxide, carried on the semiconductor wafer surface. These plasma etching techniques are used in the semiconductor industry because the plasma etching process involves low temperatures. Low temperatures are realized because the power required to generate the plasma is low. Because the process uses low temperatures the use of photoresist as a mask material is permitted. Also, because plasma etching is fundamentally a chemical process, selectivity in etching is very high and mask erosion can be made minimal.
The difficulty with such etching is non-uniformity of the species forming the plasma with respect to the material being treated. This is caused, in part by electrical field variations between the cathode and the anode.
Traditionally the products to be etched, such as wafers, are batch etched, that is they are evenly spaced upon one electrode of the plasma etching machine and exposed to the plasma. Because large numbers of wafers can be treated at once in such batch etchers they are the preferred production etching tool even though the poor uniformity realized in such tools requires considerable tradeoffs in device size, tolerances, etc. Although such batch etching increases the throughput it has been found that because of the gas flow, temperature, electric fields, plasma formation, etc. in the usual apparatus the plasma affects each wafer, in the batch, differently depending on its location in the apparatus. Thus the rate of and amount of etching realized by each wafer depends significantly on the wafers position on the electrode. In general, the etching rate varies with the radial position of the wafer on the electrode. Thus batch etched wafers are non-uniformly etched.
This problem of non-uniformity has been addressed in the past by; selectively spacing the wafers from the electrode with quartz spacers; providing extension shields around the entire electrode in an attempt to even out the dark space over the electrode; curving the electrode surface and by; altering the distribution of the plasma forming gas flow within the apparatus.
Even with these solutions, in the prior art considerable etching uniformity problems were encountered and batch uniformity was found to range 18% to 25% with the central wafers etching faster than the wafers positioned around the edges of the electrode.
Thus the solutions discussed above did not provide significant improvements to the etching uniformity problem and until the present invention the best uniformity between wafers was achieved only when each wafer was etched individually, for example, in a single wafer etcher. Such single wafer etchers have a very low throughput and therefore are expensive to use and have such a low output that their use as a production tool is economically discouraged.